Method of making doped silicon spheres

ABSTRACT

Solar cells are formed of semi-conductor spheres of P-type interior having an N-type skin are pressed between a pair of aluminum foil members forming the electrical contacts to the P-type and N-type regions. The aluminum foils, which comprise 1.0% silicon by weight, are flexible and electrically insulated from one another. The sphere are patterned in a foil matrix forming a cell Multiple cells can be interconnected to form a module of solar cell elements for converting sun light into electricity.

This application is a continuation of application Ser. No. 07/387,677,filed Jul. 31, 1989, now abandoned.

"METHOD AND APPARATUS FOR CONSTRUCTING A FOIL MATRIX FOR A SOLAR CELL",by M. J. Jensen et al., filed Jul. 31, 1989 U.S. patent application Ser.No. 387,250, now U.S. Pat. No. 4,992,138.

"METHOD OF AFFIXING SILICON SPHERES TO A FOIL MATRIX", by G. B.Hotchkiss, filed Jul. 31, 1989 U.S. patent application Ser. No. 388,105,now U.S. Pat. No. 5,091,319.

"METHOD OF ISOLATING SHORTED SILICON SPHERES", by S. G. Parker et al.,filed Jul. 31, 1989, U.S. patent application Ser. No. 387,244, now U.S.Pat. No. 5,192,400.

"SOLAR CELL WITH FOIL CONTACT POINT AND METHOD FOR ITS MANUFACTURE", byG. B. Hotchkiss, filed Jul. 31, 1989, U.S. patent application Ser. No.388,280 now U.S. Pat. No. 5,028,546; and

"METHOD FOR APPLYING AN ORGANIC INSULATOR TO A SOLAR ARRAY", by M. D.Hammerbacher, filed Jul. 31, 1989 U.S. patent application Ser. No.387,929, now U.S. Pat. No. 5,086,003.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to solar arrays and more particularlyto a method of making doped silicon spheres for use in a solar array.

BACKGROUND OF THE INVENTION

A number of systems for converting sunlight to electricity are known.One such system that has proven useful in efficiently producingelectricity from the sun's radiation is described in U.S. Pat. No.4,691,076. In that system, an array is formed of semi-conductor spheres.Each sphere has a P-type interior and an N-type skin. A plurality of thespheres are housed in a pair of aluminum foil members which form thecontacts to the P-type and N-type regions. The foils are electricallyinsulated from one another and are flexible. Multiple arrays can beinterconnected to form a module of solar cell elements for convertingsunlight into electricity.

In order to produce sufficient quantities of the arrays, it is necessaryto have a process for their manufacture that is uncomplicated, low costand efficient. An uncomplicated system would be one using currentlyavailable technology constructed in such a way that the applicableprocess steps can be conducted in a highly repeatable manner. Moreover,the less complicated the process steps, generally the more costeffective will the entire process be carried out. Finally, the morerepeatable the process, the more efficiently the operation and thehigher production of solar arrays.

A key process step in the making of silicon solar cells is the abilityto introduce controlled quantities of dopant impurity atoms into thesilicon. In one widely used method of introducing impurities into thecrystal, the impurity is delivered to the surface in vapor form dilutedto the proper concentration with an inert carrier gas such as nitrogen.While this method works quite well for planar solar cells, it hassignificant disadvantages when applied to spheral solar cells.

One significant problem with using vapor deposition for diffusingsilicon spheres is the difficulty in obtaining a uniform diffusiondepth. The spheral shape and small size (0.0175 inches in diameter) makeit very difficult to prevent the spheres from touching either otherspheres or the quartz diffusion boats. If the spheres do touch oneanother or the quartz boat, that part of the sphere would be shieldedfrom the doping gas resulting in a hondiffused area and an electricallyshorted sphere. The entire sphere surface must be doped, not just thefrontside as in planar solar cells and integrated circuit components.This shielding effect would also limit the amount of spheres diffused ina single run.

Past attempts at obtaining nonshorted spheres have included the rotationof the spheres during the vapor deposition process. This rotation didhelp, but resulted in nonuniform diffusion profiles and did notcompletely eliminate electrically shorted spheres. Nonuniform diffusionprofiles will also cause a variation in electrical properties fromsphere to sphere.

In an N on P solar cell, POCl₃ is the preferred choice as the N-typedopant due to its phosphorus concentration vs. depth profile near thesurface. Unfortunately POCl₃ becomes tacky during a diffusion run andwill deposit onto the walls of the quartz furnace tube. This residuePOCl₃ must be removed regularly to maintain cleanliness and processcontrol.

Tests have also shown that static electricity causes the spheres toadhere to the sides of the rotating cylinder preventing the spheres fromrotating and receiving a uniform diffusion.

It is also difficult with the vapor deposition process to reliablyobtain the shallow junction depths (0.2-0.3 microns) necessary forgenerating high currents and correspondingly high solar-to-electricalefficiencies. The total diffusion time, approximately 5-8 minutes at850° C.) involved in doing shallow junctions is too short for optimalcontrol and repeatability. The diffusion time becomes even shorter athigher temperatures which further complicates the control problem.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method ofproducing a solar array having the noted advantages in that products areefficiently produced using an uncomplicated and low cost process.

The method of this invention calls for immersing and agitating thespheres in a solvent-based dopant which upon baking at 150° C. dries toa highly doped silicon oxide. After the silicon oxide layer is formed,special precautions to rotate the spheres are not necessary as theentire surface area is exposed to the dopant through the silicon oxide.There is no shielding problem with the highly doped silicon oxides.Large quantities of spheres in multiple layers can be diffused at onceas the piling of spheres on top of each other does not hinder the solidstate diffusion process. The impurity contained in the glass is driveninto the sphere at commonly used diffusion temperatures of 850°-1200° C.The immersion and agitation of the spheres ensures that all parts of thesphere are coated with the silicon oxide and, thus will preventelectrical shorts from occurring.

In one embodiment of the invention, a method for doping silicon spheres,for use in a spheral solar cell, comprises a number of steps. The firststep calls for processing raw silicon oxide to produce silicon spheres.In the next step the silicon spheres are saturated in a solvent baseddopant. Next, the saturated silicon spheres are heated for a timesufficient to affix the dopant to the spheres. In the next step, thedopant is diffused into the spheres. Finally, the doped silicon spheresare processed to remove excess material from the outer surface of thespheres.

A technical advantage of this invention is that controlled quantities ofdopant impurities are introduced into the silicon spheres in anuncomplicated, low cost and efficient manner.

Other technical advantages of the highly doped silicon oxide processover the vapor deposition process for the diffusion of silicon spheresare many. Another technical advantage is that there are no shieldingeffects since the entire sphere surface is exposed to the dopant throughthe silicon oxide, which completely surrounds the sphere. Still anothertechnical advantage is that uniform junction depths over the entiresphere surface are obtained. Another technical advantage is that ashallow junction depth of 0.2-0.3 microns is repeatedly and reliablyobtained. More uniform sheet resistivity values translate into onlyminor variations in the electrical properties from sphere to sphere.

Another technical advantage is a process which is much more forgivingand less sensitive to time and temperature fluctuations. Still anothertechnical advantage is a process which requires much less furnacemaintenance. Another advantage is better control over surface impurityconcentrations (i.e., just lower or raise the impurity level in thesilicon oxide to control the surface concentration). Still anothertechnical advantage is the shortened production time as largerquantities of spheres can be diffused in a single run. Finally, byrepeating the process at least two times in can be practiced withoutusing a clean room.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a solar cell after each of theprocessing steps utilized in accordance with the present invention;

FIG. 2 is a process schematic diagram of the process utilized in forminga solar array in accordance with the present invention;

FIG. 3 is a process schematic diagram of the process for producing dopedsilicon spheres in accordance with the present invention;

FIG. 4 is an apparatus for embossing aluminum foil in accordance withthe present invention;

FIG. 5 is an apparatus for use in accordance with the present inventionfor loading silicon spheres and constructing aluminum foil cellpackages;

FIG. 6 illustrates the cell package layers used in accord with thepresent invention;

FIG. 7 is a set of diagrams illustrating various foil layout patternsfor use with the front bonding process;

FIG. 8 illustrates the positioning of pressure pads in the loadingstation of the present invention;

FIG. 9 is a completed cell package for use with the present invention;

FIG. 10A shows a hydraulic press for binding silicon spheres to aluminumfoil and for affixing aluminum pads to the silicon spheres in accordancewith the present invention;

FIG. 10B is a diagram of the hydraulic press in FIG. 10A modified foruse with the invention;

FIG. 11 is a schematic diagram of a process in accord with the presentinvention for selective electro-dissolution of shorted silicon spheresin a solar array;

FIG. 12 illustrates the package layers used in affixing the aluminumpads in accordance with the present invention; and

FIG. 13 is an applicator for applying an organic insulator to a solararray in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a solar cell resulting from thevarious processing steps utilizing the features of the presentinvention.

Initially as shown in FIG. 1(a), a flexible aluminum foil 2, with from0.5% to 1.5% but preferrably 1.0% silicon by weight, of about two milthickness is provided. Foil 2 has a very thin oxide layer on its surfacedue to normal exposure to the environment. While the description hereinwill be with respect to a single solar array member or cell, it shouldbe understood that a multiplicity of array members is provided in thetotal array as is exemplified by the prior art noted herein above.

The aluminum foil 2 is initially embossed in a periodic hexagonalarrangement, on sixteen mil centers. The diameter of the embossed regionis slightly smaller than the diameter of the doped spheres 4 to bedisposed therein. The embossments can be circular or of any othergeometrical shapes such as hexagonal or octagonal. In the case ofpolygonal shape of the embossment, a line across the polygon through itscenter will be less than the diameters of the spheres to be appliedthereto. The foil 2 is then cleaned to remove organics and then etchedwith heated sodium or potassium hydroxide to remove the region of thefoil where the embossing is made to provide an aperture 6 in its place.The etched foil comprised of a plurality of apertures 6 is referred toas an aluminum matrix.

At this point the foil can optionally be textured by etching with afifty percent solution of 39A etchant which is twenty-five percent HF,sixty percent HNO₃ and fifteen percent glacial acetic acid to provide amatrix surface that minimizes back reflections.

The doped silicon spheres 4 are deposited over the frontside 14 of thematrix on foil 2 and vacuum is provided on the backside 16 of the foilwith a vacuum chuck to draw the spheres 4 into the apertures 6. Becausean excess of spheres 4, relative to the number of apertures 6, isinitially utilized on the foil frontside 14, all of the apertures 6 willbe filled with the spheres 4 and the excess spheres 4 are then removedby known techniques.

The spheres utilized are preferably 14.5 mils in diameter and theapertures 6 as explained above, have a cross-sectional diameter ofsomething less than 14.5 mils to provide a vacuum with the foil at thefoil front 18 for reasons to be made clear hereinafter. The spheres 4are then bonded with the aluminum foil 2 within the apertures 6 as shownin (b).

Referring to FIG. 1(b), the sphere 4 is disposed in the aperture 6 sothat the equator thereof is forward of the aluminum foil 2 or on thefront side 14 thereof. This arrangement is made possible by the use ofpressure pads which are disposed above and below the aluminum foil 2.Pressure pads are formed of aluminum foil from about 1 mil to about 8mils thick and coated with a release agent, such as boron nitridepowder, which acts as a cushion so that the press does not injure thespheres 4 during compression.

The rear surface 16 of the foil 2 and the portion of sphere 4 on therear side surface is then etched using any of planar, HF/HNO₃ /urea or39A etchant (approximately fifteen percent glacial acetic acid,twenty-five percent HF and sixty percent HNO₃). As shown in FIG. 1(c),the N-type layer 10 on the back surface of sphere 8 is removed exposingthe P-type region 12. The aluminum foil 2, with the native oxidethereon, acts as a mask to the etchant permitting only the portion ofthe layer 10 to the rear side of the array to be removed. The array isthen rinsed with the deionized water to remove the etchant. As shown inFIG. 1(d), a polyimide coating 20 such as Kapton® available from DuPontis administered to the backside 16 of foil 2.

In FIG. 1(e), polyimide coating 20 on backside 16 is abraded to remove asmall region 22 of polyimide coating 20 to expose a portion of theP-type material 12 of sphere 4.

In the next step, the array undergoes an anodizing process to isolateshorted cells. In the process, the array is immersed in a diluted HFbath with a potential difference of approximately 0.5 volts between theanode and cathode. The anodization process takes approximately thirtyseconds providing a sufficient insulating coating between P-region 12 ofsphere 8 thus isolating a sphere 8 that is shorted to foil 2.

In FIG. 1(f), an aluminum pad 24 is affixed to the P-region 12 throughregion 22 of sphere 8 to form an electrical connection 26.

In FIG. 1(g) a thin aluminum foil 28 of about 0.5 mil thickness is thenpositioned over the aluminum pad 24 of each of the spheres 8 so that itwill contact the aluminum pad 24 at electrical connection 26. Thealuminum is heated to a temperature of about 400° C. preferably, and inthe range of about 350° to 450° C. The heated foil is then pressedagainst the array by means of a compression press such as that describedhereinafter forming a contact between the aluminum pad 24 and the foil28.

Referring now to FIG. 2, a process for constructing the solar cell shownin FIG. 1 is described. Initially, raw silicon oxide 40 is processed,using known techniques, at rounding station 42 to produce siliconspheres 44. Techniques for rounding the silicon spheres are shown inU.S. Pat. Nos. 4,425,408, 4,637,855, and co-pending U.S. patentapplication Ser. No. 188,184, assigned to the Assignee of the presentapplication now abandoned. The silicon spheres 44 are subjected to adoping process at doping station 46 to produce doped silicon spheres 48having an N-type skin and a P-type interior.

The doped silicon spheres 48 are then loaded on the apertured aluminumfoil at load station 50. Prior to loading the doped silicon spheres 48at load station 50, the raw aluminum foil is embossed with the matrixpattern at emboss station 47 and the apertures etched into the foil atetch station 49. The foil with the loaded silicon spheres 48 is moved tobond station 52 where a compression bonding technique describedhereinafter is used to bond the doped silicon spheres 48 to the aluminumfoil as depicted in FIG. 1(b).

At etch station 54 the N-type region of silicon sphere 48 is etched onthe backside of the solar array to expose the P-region of the sphere asshown in FIG. 1(c). In the next step, a polyimide is applied at coatingstation 56 to provide an insulating layer as depicted in FIG. 1(d).

At shunt station 58, a portion of the polyimide coating is removed toprovide a region for fixing the aluminum pads as described hereinafter.After abrading and removing the polyimide at etch station 60, the arrayis subjected to an anodizing process at anodize station 62 to isolatethose spheres that have a P-region shorted to the aluminum foil orotherwise forming a uninsulated contact. After the shorted spheres areshunted at shunt station 58, the aluminum pads are affixed at bondstation 64. After the aluminum pad are affixed at bond station 64, thealuminum foil backing is applied at backbond station 66 forming acomplete solar cell such as that depicted in FIG. 1(g). The completedsolar cell is conveyed to test station 68 where various tests, as iswell known to those skilled in the art, are performed.

Referring now to FIG. 3, the method for preparing the doped siliconspheres is described. Raw silicon oxide 80 is processed using knowntechniques to form silicon spheres 82. The silicon spheres are loaded ina quartz boat 84 which is filled with dopant taken from either Group IIIor Group V of the Periodic Table of Elements dissolved in a liquidsolvent-carrier to form a solvent-based dopant. In the preferredembodiment, phosphorus dopant available from Diffusion Technologies ofMilipitas, Calif., is used. The excess solvent-based dopant 86 is drawnfrom quartz boat 84 so that the spheres 82 are barely covered by theliquid. The saturated silicon spheres are allowed to air dry for between20 and 90, preferably 60, minutes and then heated in an oven 88 at atemperature in the range between 100° C. and 200° C. for between 15 and60 minutes, preferably 150° C. for 30 minutes. After heating, thesaturated silicon spheres are taken to a quartz diffusion tube 90 andheated in a controlled atmosphere such as a nitrogen atmosphere at atemperature in the range of 850° C. to 1200° C. for approximately onehour. After the diffusion step, a thin film of phosphorus glass 92 isformed on the outside of the silicon spheres. That phosphorus glass isremoved using a rinse 94 comprised of at least a ten percentconcentration of HF. After etching off the excess phosphorus glass 92,the silicon spheres 82 are rinsed in deionized water 96 for thirtyminutes. In the next step, the silicon spheres are spun dry in dryer 98for between 200 and 800 seconds to remove excess moisture. The spheresare then resaturated and the process repeated once again to provideadequate penetration of dopant into the raw silicon oxide. The range ofpenetration is between 1.00 microns and 1.50 microns with 1.25 micronsappearing to be optimally obtained after two passes through the process.

Referring now to FIG. 4, a book rolling apparatus 100 for embossing thealuminum foil to produce apertures for accepting the silicon spheres isdescribed. A thin sheet of aluminum foil 102 of between 2.0 and 3.0 milsthickness is sandwiched between a flexible cover plate 104 and anembossing tool 106. The sandwich 107 is fed between two rotating rollers108 and 110 which create a series of depressions or regions of lesserthickness in the aluminum foil.

The flexible cover plate 104 is constructed of spring steel with athickness substantially greater than the thickness of aluminum foil 102,about 0.125 inches thick. Cover plate 104 is chamfered at the edges tominimize the contact angle with the top roller. Embossing tool 106 isconstructed of hardened steel, tungsten carbide or other hardenedmaterial (R_(c) ≧60) with outside dimensions of 3.0"×3.5"×0.5" thick. Itmust be built to very strict tolerances and each post must be square tothe top face 113. In the preferred embodiment, embossing tool 106 isconstructed using electron discharge machining. The posts 112, which dothe actual penetration into the foil, number about 5100 and can beeither round or polygonal but must have very smooth faces 116. Each post112 has selected dimensions so as to produce a region of lesserthickness in foil 102 that when etched away will produce an aperturewith a diameter less than the silicon spheres. The layout or pattern ofposts 112 is selected to maximize the number of post per area. Posts 112are centered with respect to the edges of embossing tool 106 and arecontained within a square having side dimensions of 1.25 inches. Thealuminum foil 102 is doped with 1% silicon by weight.

Rollers 108 and 110 are separated from one another forming a nip 114having a dimension that is less than the thickness of cover plate 104,foil 102 and embossing tool 106. In the preferred embodiment, sandwich107 is passed through roller 108 and 110 twice in order to obtainsufficient penetration of posts 112 into foil 102. Each pass requiresabout two seconds to feed sandwich 107 completely through rollers 108and 110. Sandwich 107 can be fed to rollers 108 and 110 by suitablefeeding means, such as a conveyor belt (not shown but well known tothose skilled in the art).

In the preferred embodiment, nip 114 is selected to provide anapproximate average pressure in the range of 16000 lbf to 20000 lbf withapproximately 18000 lbf being optimum.

In practice, the roller 108 makes point contact with posts 112distributing the above mentioned pressure across several rows. In thepreferred embodiment, roller 108 would span only one row of posts 112thus maximizing the pressure. It is recognized that the above describedpressures will vary according to the conditions of the embossing. Forexample, a wider embossing tool for larger sized cells will require moreforce to maintain the same pressure and should scale as the ratios ofwidths. A larger diameter roller would also require proportionally moreforce.

After the embossing is complete and the foil has regions of lesserthickness thereon it is etched at 60° C. with 45% KOH etchant to removethe regions of lesser thickness leaving a foil matrix with patternedapertures. Higher KOH temperature can result in foil etching too fast incertain areas and creating "hot spots" due to high concentrations orpools of silicon in those areas.

After the apertures are etched into the foil and the silicon spheres aredoped, the next step is to prepare a cell package. The first step inloading the spheres is to cup the front foil. This technique was foundvery helpful in preventing the outermost spheres from moving before thebond is made. An 8 mils thick aluminum foil, though other thicknesseswork as well, with a square cutout in its center is placed on the cell'sfrontside. The cutout is just larger than the length and width of thecell pattern. The "cupping mask" is carefully placed around theperiphery of the embossed area. By applying a downward force to the 8mil foil, the front foil near the edge of the opening is pushed downwardforming a ridge or cup all the way around the embossed area.

The spheres are loaded onto the front foil by means of a vacuum chuck.The vacuum chuck used a porous plastic material such as that from PorexTechnologies, Fairburn, Ga., as the diffuser rather than a metal screen.The porous plastic provides a much more uniform vacuum and eliminatesthe blockage of foil holes by the web of the metal screen. The vacuumchuck containing the front foil is held over a collection box while thespheres are poured onto the foil until the embossed areas is completelycovered. The vacuum exerts enough downward force to attract and hold thespheres in the holes. Any excess spheres such as those on a second orthird layer and around the periphery can be jarred loose by tilting andagitating the vacuum chuck. The loose spheres fall into the collectionbox for use on subsequent cells.

After the sphere loading is complete, the foil is then transferred to aloading station. The loading station, depicted in FIG. 5 is a 3-tonArbor press (Dake Model #001). The two platens 120 and 122 are made ofaluminum with silicone rubber glued to their exposed surfaces at 124 and126. The top platen 120 is 2.5"×2.5"×0.5"thick with 0.125" thick rubber.The bottom platen 122 is also 2.5"×2.5"×0.5" with 0.125" thick rubber.Both platens were machined to provide a vacuum cavity (not shown).Referring to FIG. 7, before the foil with the loose spheres istransferred to the loading station, two 3"×3"×0.008" thick coatedaluminum foils 128 and 130 are positioned on the bottom platen 122 in adiamond shape (turned 90° off axis) such that their corners are extendedover the four edges of the platen as shown in (A).

Referring to FIG. 7(b), a second 3"×3"×0.008" thick coated aluminum foil132 is then placed on top of the bottom foils 134. The edges of the topfoil are parallel to those of the platens of the press. The coated sidesof the foils face each other. The top plate is lowered and the vacuum isapplied to the platens. The corners of the bottom coated foil aresquared making it easier to fold the tabs over the top coated foil asshown in FIG. 9. Once the tabs are cut, the top platen is raised takingwith it the topside coated foil. FIG. 8 shows the 8 mil thick coatedaluminum foils 128 and 130, which are called release pads or pressurepads, positioned in the loading station.

The front foil 136 is removed from the vacuum chuck and placed betweenthe two coated foils 132 and 130 as is shown in FIG. 8. The top plate120 is lowered until contact is made with the cell 138 and the tabs 140folded up and over the topside foil 132 to form one cell package (FIG.9). The pressure from the press is maintained on the cell package 138until the tabs 140 are folded. The vacuum on the plates is then releasedand the top plate 128 withdrawn. The rubber 124 on the top platen 120deforms the 8 mil coated Al foil around the spheres which along with thecupping of the front foil prevents the spheres on the outermost rowsfrom moving.

The foil with the spheres positioned on the backside of foil 2 in FIG.1(a) is removed from the vacuum chuck and placed between pressure padssuch as a multiple layer 8 mil thick coated aluminum foil package shownin FIG. 6 by folding the two package ends on top of the cells. The cellpackage is then placed on the bottom platten of a precision press. Thebottom platten is slowly raised until the top platten is contacted. Thetwo plattens are left in contact with the solar cell sandwiched inbetween for a period of time not exceeding 1.5 minutes. After the elapseof 1.5 minutes, the plattens are then separated and the cell is removed.Both the top and bottom plattens are maintained at a temperature of 500°C. The aluminum of foil 2 reacts with the very thin native silicon oxidelayer on spheres 8 and removes it so that the aluminum in the foil 2 isnow able to bond directly to the elemental silicon in the N-type layer10 of sphere 8 to form a contact thereto.

The completed cell package, shown in FIG. 9, facilitates thetransporting of the cell without movement of spheres from the loadingstation to the front bonding press.

Referring now to FIG. 6, the front bond cell sandwich 150 is constructedas follows. A cell package 152 comprises an 8 mil thick raw 1145aluminum foil 154 and a second 8 mil thick 1145 aluminum foil 156 havinga release coat 158. Release coat 158 and the various release coatshereinafter discussed are constructed by mixing 20 ml printers' ink, 20ml ethylene-glycol, 6.0 gms of boron nitride and 6.0 gms of silica;additional ethylene-glycol can be mixed in to the foregoing to achieve adesired consistency. Next to release coat 158 is foil matrix 160 withspheres 161 resting thereon. Mounted on top of spheres 161 is 1 milthick 1145 aluminum foil 162 with release coat 164 facing the top ofspheres 161. Foil 166, also 8 mil 1145 aluminum with a release coat 167,completes cell package 152.

Cell package 152 is pressed between upper pressure pad 168 and lowerpressure pad 170. Upper pressure pad 168 is comprised of foil 172, an 8mil thick 5052 aluminum foil, with a release coat 174 facing cellpackage 152. A layer 176 of compressed graphite, such as Grafoil® fromUnion Carbide, s upper pressure pad 168. Lower pressure pad 170 iscomprised solely of compressed graphite layer 178 which can be the sameas layer 176.

Foil 162 of cell package 152 is constructed of aluminum having between0.5% and 1.5% but preferrably 1.0% silicon by weight. As the variouslayers are compressed, spheres 161 are forced into apertures 163 as haspreviously been explained.

A bonding press 240 FIG. 10(A) is shown schematically in FIG. 10(B).Press 240, is a four-poster made by Carver of Menomonee Falls,Wisconsin, Model No. 2629-X and has the capability of delivering 30 tonsforce at 600° C. However, for bonding silicon spheres to aluminum foil,the press has to be modified to deliver fairly large forces (12.5-15tons) during a very short time span (0.10 sec.) and over smalldisplacements (0.050-0.100 inch). To accomplish this end, a hydraulicintensifier 250, accumulator 252, and pneumatically actuated mechanicalvalve 254 were added to the press. FIG. 10 shows a piping schematic ofthe modified press. These add-on items are standard off-the-shelf itemsavailable from most hydraulic component manufacturers as is well knownto those skilled in the art.

The hydraulic intensifier 250 is from Haskell of Burbank, Calif. The airdriven fluid pump, Model AW-35, delivers maximum liquid pressure of 5700psig at an air pressure of 150 psig. The accumulator 252 is from UnitedTechnologies Diesel Systems of Springfield, Mass. Model ACH-5A-08-P-NT,having capacity of 5.0 in ³, with a working pressure of 5000 psig.Mechanical valve 254 is Model 20395N (pneumatic with spring return) fromWorcester Controls of West Boylston, Mass.

Mechanical valve 254, Model IH446YTSE, 1.0" having a 0.81" port withmaximum pressure of 4500 psig, was selected over various solenoidcombinations. The reason is that in order to charge up accumulator 252without disturbing press ram 256, a leak-tight valve was necessary whichthe solenoid valves could not provide. The mechanical valve 254 was alsooversized to ensure that sufficient fluid flow began as soon as thevalve 254 was only partially open.

It is important that the platens 241 and 242 (FIG. 10A) be alignedparallel to each other. This is accomplished by adjusting the top platen241 which in the Carver press moves up and down on four threaded rods243 and is held in check by nuts 244. The bottom platen 242 travelsalong the threaded rods 243 as well but is not adjustable. Theparallelism of the platens 241 and 242 is checked at room temperature bymeasuring the change in thickness of 0.080" diameter solder shaped intoa 1.50" square. The solder square was placed inside a hinged plate andthen pressed using the standard front bond cycle. Any significant(0.001") deviation of the four sides relative to each other wascompensated for by lowering or raising that particular side of the topplaten 241.

Another more practical method of aligning the press platens was to bonda back foil to a cell and observe the following: (1) the impressionsmade on the frontside pressure pads and (2) the size and uniformity ofthe aluminum pad. For example, if the spheres penetrate the pressurepads deeper on one side, obviously this side of the top platen 241 needsraising relative to the others. Likewise an aluminum pad with largersurface areas in one particular region of the cell indicates excessivepressure being applied there. The advantageous feature of this alignmentmethod is its in situ approach. The alignment is measured under actualtest conditions and on a cell, not a piece of solder at roomtemperature. The disadvantage is that it is a destructive test becausethe cell used for the test cannot or should not be used again except forfuture alignment checks.

The standard cycle of press 240 for a 10 cm² bond is as follows. Thecell package 152 (FIG. 6) and sandwich 150 were placed in a preheatedhinged plate 151 which consists of two pieces of a hardened materialsuch as tungsten carbide or hardened steel (R_(c) =62), 4"×4"×0.5"thick, joined at the back by a hinge. Hinged plate 151 is open at thefront. The four faces of the plates 151 were Blanchard ground flat towithin±0.0001" and parallel to ensure uniform pressure across the cell.

If the hinged plate is not used the cell package parts will tend to bowor warp at the edges due to the thermal expansion differences whenplaced on the platens to preheat. This movement of the aluminum pressurepads will cause the spheres to move out of their holes in the front foilresulting in misplaced spheres and empty holes after bonding. The hingedplate resolves the expansion problem.

Cell sandwich 150, set between hinged plates 151 is pushed forward untilit is centered over the ram 256. The bottom platen 242 is then slowlyraised until hinged plate 151 makes contact with the top platen at aforce of between 0.5 and 1.5 tons.

After a preheat dwell period of between 0.5 to 2.0 minutes, accumulator252 is charged to 6000 psig with hydraulic intensifier 250. Thischarging sequence requires about 15 seconds. Previous to this charging,mechanical valve 254 is closed isolating ram 256 from the accumulator.

As soon as accumulator 252 is fully charged, an electrical switch (notshown) is activated which opens the inlet solenoid valve (not shown) toactuator 258 opening mechanical valve 254. As soon as valve 254 startsto open, accumulator 252 instantaneously tries to discharge moving ram256 in the range between 0.5 and 1.5 tons preheat force to the finalbonding force of between 12.5 and 15.0 tons, or between 5.0 lbs. and 6.0lbs. per sphere, in less than 0.1 second. After between 1.0 and 60.0seconds, the platens 241 and 242 are released and the cell package isremoved.

After the front bonding process described with respect to FIGS. 6 and 10is complete and the spheres have been bonded to the foil, the rearsurface of the foil and that portion of the sphere on the backside isthen etched, as is well known to those skilled in the art, to remove theportion of the N-type layer on the backsurface of the array and exposethe P-type region. After the N-type layer has been removed it isnecessary to test the solar array to identify and isolate those cellshaving a low resistance path across the P-N junction. The process forshunting the low resistance cells comprises a first step of applying apolyimide coating to the backside of the foil and the P-type region ofthe solar sphere.

Referring to FIG. 13, an apparatus for applying an organic insulatingmaterial, such as polymide, is depicted. Fixture 300 has jaws 302 forholding solar array 304 in a sealing arrangement to base 306. Base 306is plumbed for applying pressurized gas in the range between 1 and 5PSI, such as compressed air or nitrogen, to the light-gathering side 305of solar array 304. A gas supply (not shown) would be connected to inletport 308 and conveyed along tube 310 to outlet port 312.

Still referring to FIG. 13, sprayer 314 is a conventional atomizercontaining a liquid polymide 316. The atomized polymide is applied tosolar array 304 through spray nozzle 318. The pressure built up betweenbase 306 and array 304 prevents the organic insulating material fromleaking through holes in array 304 onto the light-gathering side 305.This results in a solar array with a uniform dielectric coating on thebackside, and no coating on the light-gathering side.

After the polyimide coating has been applied, covering the entire backsurface of the foil and the solar cells, a small amount of the polyimideis removed from each solar cell by mechanical abrading. Only a smallamount of the polyimide is removed at the tip of the solar cell leavingsufficient area for subsequent attachment of the aluminum pads. Afterthe mechanical abrading is completed the polyimide is cured, using knowntechniques, to provide a hard insulating coating everywhere on thesurface except where the sphere has been abraded.

Referring now to FIG. 11, the solar array 190 is immersed in an acidbath 192. Bath 192 consists of hydrofluoric acid (HF) and H₂ O in amixture of at least 45% HF. It has been found that at least 52% HF isoptimal. Plate 194 is maintained parallel to array 190 and can either bea solid plate or screen. Plate 194 should be of approximately the samearea as cell 190 and preferably larger. The distance between plate 194and array 190 should be approximately equal to the width 196 of array190. The potential difference across plate 194 and array 190 should beapproximately in the range of 0.20 to 1.00 volts and preferably 0.55volts.

The anodic process depicted in FIG. 11, if conducted at 0.55 volts,should be maintained for between 10 seconds and 5.0 minutes. At lowervoltages the anodic process should be conducted for longer periods oftime. It has been found, however, that shorter times are preferable.

While the process has been conducted at room temperature, it has beendiscovered that lowering the temperature of bath 192 is preferred. Onetechnique (not shown) is to immerse acid bath 192 in a chilled waterbath to just above 0° C. Other techniques, commonly known to thoseskilled in the art, would work as well. Agitator 198 is provided to morecompletely mix the HF/H₂ O solution of acid bath 192. It should also benoted that lower HF concentrations of acid bath 192 requires a highervoltage across plate 194 and array 190.

The anodic process depicted in FIG. 11 results in a silicon oxidecoating being deposited on those solar cells that have a short betweenthe P-N junction. This effectively removes the cell from the arraycircuit and avoids a resulting power drain. After the shorted cells havebeen isolated the solar array is ready for receiving the aluminum pads.

The same hydraulic press used for making the front bond is also used forthe aluminum pad installation. A cell package is constructed andinserted between a pair of hinged plates and pressure applied to performthe junction between the aluminum pad and the P-type region of the solarsphere.

Referring now to FIG. 12, there is shown a cell sandwich 199 for use ininstalling the aluminum pads. Array 200 with voids 202 abraded into thesurface of polyimide coating 204 is sandwiched between upper layer 206and a bottom layer 208. Upper layer 206 has thick aluminum foil 210which is about 8 mils thick and has release coating 212. Against releasecoating 212 is a thinner aluminum foil member 214 having release coating216. Release coating 216 faces front side 218 of array 200. In analternative embodiment (not shown) a layer of compressed graphite can beadded to upper layer 206.

Still referring to FIG. 12, a pressure pad such as bottom layer 208comprises thin foil member 220, having between 0.5% and 1.5% butpreferrably 1.0% silicon by weight, approximately 0.25 mils thickaluminum foil. Beneath foil member 220 is plate 222 which is a 2 milthick stainless steel plate having release coating 224. Cell sandwich199 is set between a pair of hinged plates 226 and pressed to a snugforce of 3000 lbf, or between 0.4 lbs. and 0.6 lbs. per sphere, using ahydraulic press such as that described with respect to FIG. 10. Theentire cell sandwich 199 fixed between hinged plates 226 is preheated atthe pressure 3000 lbf and a temperature in the range between 300° C. and400° C. preferably closer to 350° C. for a period of time in the rangebetween 0.5 and 2.0 minutes, preferably 1.5 minutes, to allow the celland foil member 220 to reach a steady-state temperature. At the 1.25minute stage of the preheat cycle, mechanical valve 254 (FIG. 10B) isclosed and accumulator 252 (FIG. 10B) is charged to a pressure of 2000psi. At the 1.5 minute mark, mechanical valve 254 is openedinstantaneously to a platen force of between 8000 and 9000 lbf orbetween 1.25 lbs. and 2.00 lbs. per sphere. Cell sandwich 199 is left atthis force for a dwell period in the range between 0.25 and 2.00minutes, preferably 1.00 minute, and at the end of that time, pressureis released, and the cell is removed for cooling. After cooling, theexcess of foil 220 is removed producing a solar cell such as that shownin FIG. 1(f) ready for the back bonding and testing steps as haspreviously been described.

Though the invention has been described with respect to a specificpreferred embodiment thereof, many variations and modifications willbecome apparent to those skilled in the art. It is therefore theintention that the appended claims be interpreted as broadly as possiblein view of the prior art to include all such variations andmodifications.

What is claimed is:
 1. A method for doping silicon spheres with an outersurface comprising the steps of:coating said outer surface of saidspheres by immersing said silicon spheres in a solvent-based dopant andagitating said spheres during said coating of said outer surface of saidsilicon spheres, said agitation ensuring that the entire outer surfaceof each sphere is coated with said solvent-based dopant; heating saidcoated silicon spheres to form doped silicon oxide on said outer surfaceof said silicon spheres; diffusing said dopant into said siliconspheres; and processing said doped silicon spheres to remove excessdoped-oxide from said doped silicon spheres.
 2. The method of claim 1wherein said solvent-based dopant is selected from Group III of thePeriodic Table of Elements.
 3. The method of claim 1 wherein saidsolvent-based dopant is selected from Group V of the Periodic Table ofElements.
 4. The method of claim 3 wherein said solvent-based dopantcontains phosphorus.
 5. The method of claim 1 further comprising thestep of air drying said coated silicon spheres prior to the heatingstep.
 6. The method of claim 5 wherein said coated silicon spheres areair dried for a time period in the range between 20 and 90 minutes. 7.The method of claim 1 wherein said coated silicon spheres are heated toa temperature in the range between 100° C. and 200° C.
 8. The method ofclaim 7 wherein said coated silicon spheres are heated for a time periodin the range of 15 to 60 minutes.
 9. The method of claim 1 wherein saiddiffusing step is conducted in a quartz diffusion tube.
 10. The methodof claim 9 wherein said diffusing step is conducted in a controlledatmosphere.
 11. The method of claim 10 wherein said controlledatmosphere consists essentially of nitrogen.
 12. The method of claim 10wherein said diffusing step is conducted at a temperature in the rangebetween 850° C. and 1200° C.
 13. The method of claim 1 furthercomprising repeating said coating, heating, diffusing and processingsteps to obtain a predetermined junction depth.
 14. A method forproducing doped silicon spheres comprising the steps of:providingsilicon spheres with an outer surface; coating said outer surface ofsaid spheres by immersing said silicon spheres in a solvent-based dopantand agitating said spheres during said coating of said outer surface ofsaid silicon spheres, said agitation ensuring that the entire outersurface of each sphere is coated with said solvent-based dopant; heatingsaid coated silicon spheres to form doped silicon oxide on said outersurface of said silicon spheres; diffusing said dopant into said siliconspheres; and processing said doped silicon spheres to remove excessdoped-oxide from said doped silicon spheres.
 15. The method of claim 14wherein said solvent-based dopant is taken from Group V of the periodictable of elements.
 16. The method of claim 14 wherein said solvent-baseddopant is taken from Group III of the periodic table of elements. 17.The method of claim 14 further comprising the step of air drying saidcoated silicon spheres prior to the heating step.
 18. The method ofclaim 17 wherein said coated silicon spheres are air dried for a timeperiod of 20 to 90 minutes.
 19. The method of claim 14 wherein saidsilicon spheres are provided in a single layer.
 20. The method of claim14 wherein said silicon spheres are provided in multiple layers.